Many modern cellular communication systems, such as High Speed Pack Access (HSPA) and Long Term Evolution (LTE), use automatic link adaptation to achieve efficient communication under varying radio conditions. The effective bit rate is varied quickly depending on the radio conditions. As the effective bit rate varies also related transmission parameters, such as code rate and modulation scheme, are varied. When the radio conditions become worse, the bit rate is decreased to reduce the probability of decoding error at a receiver. When the radio conditions become better, the bit rate is increased to increase the transmission throughput without causing a too high error probability. The radio conditions are often predicted based on past measurements on a radio channel for which the radio conditions are to be estimated.
When the receiver fails in its attempt to decode a transport block, it typically stores the received signal (or a processed version of it), and combines it with a later received signal for a retransmission of that block. This is known as soft combining. The soft combining greatly increases the probability of a correct decoding. Known variants of soft combining are Chase combining and incremental redundancy.
In many Hybrid Acknowledgement Response reQuest (HARQ) protocols, the receiver sends HARQ feedback, after each decoding attempt, in the form of a positive acknowledgement (ACK) or negative acknowledgement (NACK). In this manner, it is indicated to a transmitter if the particular transport block was correctly decoded or not. In case a NACK is sent, the transmitter typically retransmits the transport block. In the case of ACK, the transmitter can instead use its resources to transmit new data, to the same or a different user equipment (UE).
In both HARQ variants, the receiver must perform a complete decoding attempt before it can decide upon its next action. Modern error-correcting codes, such as Turbo codes, are very complex to decode, resulting in long delays from transmission until a message can be sent back to the transmitter. This results in long round-trip delays of the HARQ retransmissions. For example in LTE, the minimum round-trip delay is 8 ms for downlink transmissions. Hence, this limits how fast retransmissions can be made and also implies a limitation in how aggressive modulation and coding can be used without losing performance. Hence, it is often optimal to use conservative link adaptation.
It is well-known that HARQ with soft combining can be viewed as a kind of (implicit) link adaptation mechanism. This is the case if the link-adaptation is chosen so high that one or more retransmissions are often needed. The effective bit rate of the entire transmission of a transport block then depends on the number of transmissions (including original and retransmissions) as well as transport format parameters such as modulation and code rate.
The effective bit rate is then a function of the link adaptation chosen for the transmission and the number of retransmissions needed.
A known cellular radio communication system comprises a radio base station and a user equipment. The radio base station obtains information about channel conditions and schedules transmissions to/from the user equipment accordingly. In scenarios, where the channel conditions vary rapidly with respect to the minimum round-trip delay mentioned above, it may be a problem that the scheduling, including amongst others link adaptation, cannot follow the variations of the channel condition. As a consequence, performance may degrade.